It goes without saying that laboratory animal such as rats and mice must be housed in a way that keeps them comfortable and healthy. Efforts are expended on them by highly trained lab personnel while performing experiments and tests, often over a long period of time. These expensive efforts make these animals individually extremely valuable. It is important both to be humane and to protect these investments that the animals have healthy and comfortable living space.
It is most convenient and cost effective to keep lab animals in high-density housing, but this has a number of consequences. High-density housing has the potential for contamination between individual animals, which can affect the outcome of experiments. High-density housing requires positive ventilation to provide uncontaminated air for breathing and to remove the heat, humidity, carbon dioxide, and other contaminants that these animals product simply in the course of existing. It is also important that humans be protected from the air exhausted from high-density lab animal housing. (This is true for lab animals in any kind of housing, but when animals are in high-density housing the potential for risk to nearby humans increases.)
U.S. Pat. Nos. 5,954,013 (Gabriel, et al.) and U.S. Pat. No. 4,989,545 (Sheaffer, et al.) both assigned to Lab Products Inc. for example show features of the current state of the art for high-density lab animal housing. ""013 and ""545 show such housing as comprising a rack storing several tens of drawers or cages in a side-by-side rectangular grid pattern. Each drawer has a floor, and back, side and front walls generally in the shape of a shoebox. Each drawer can be inserted into its operating position from a front side of the rack by sliding on guides into the rack. These guides support the drawer in its operating position.
Each drawer has an inlet for connection to an inlet plenum forming a part of the rack and providing pressurized fresh air. Each drawer typically has a discharge filter. The rack has an exhaust hood or shroud above each filter and connected to an outlet plenum that is also a part of the rack. The outlet plenum is held at a below ambient pressure by an exhaust fan, causing air within each drawer to flow through the filter to the shroud and into the outlet plenum. The pressures in the two plenums are controlled to maintain a desired pressure in each drawer. This prevents air within the drawers from reaching the ambient space where humans may be present even though there is no hermetic seal between the ambient space and the interior of the individual drawers when in their operating position. Each drawer typically houses one to five animals depending on the drawer size and the species.
The inlet plenum receives air from a blower and a HEPA filter. The exhaust plenum discharges to the ambient space, where a building exhaust system removes the exhausted air. One problem with exhausting to the ambient air is that humans sharing the space with the racks breathe air mixed with the air exhausted from the rack. Even if the exhaust air from the exhaust plenum is filtered, odors, heat, and gasses emanating from the animals remain in the filtered air. Alternatively, the exhaust can be connected directly to the building exhaust system. But in this case, varying pressures in the building exhaust system change the velocity of air passing through the drawers and affect the air temperature in the drawers. These changes can cause distress of the animals or at least affect their response to experimental activities, which may well raise questions about the validity of experiments using the animals as subjects. validity of experiments using the animals as subjects.
The animal holding systems now in use can be modified to avoid some of these problems by including a constant volume inlet valve providing fresh air to the inlet plenum. The fresh air may be provided from a source at higher pressure than room ambient. Air within the outlet plenum flows through a constant volume outlet valve to the lower pressure of an exhaust duct of the building. The building exhaust duct then conveys the air from the animals in the rack to the outside.